Why do some maps and some nurseries use a three digit zone number, some a one digit zone number?
The original mapping system used a three digit number to identify the numerous zones. In 1996 the Oregon maps were revised to reflect current research which indicated that temperature and precipitation were primary factors in genetic selection. It was discovered that the zones could be elongated north and south following these two factors and some zones could be consolidated. The new zones were labeled with a single digit. Since the three digit system is more precise and that information is still available to us we will continue to use it. However, the new information is extremely valuable when a planting needs to be completed and seed from that precise area is not available: we can know which of the neighboring areas is best suited as an alternate.
